1. Field of the Invention
The present invention relates to an image search technique based on the degree of similarity of feature points in images.
2. Description of the Related Art
In general, the image search corresponding to the rotation of an image is performed by obtaining a feature amount after the rotation of a query image, or by using a feature amount after rotational transform.
Japanese Patent Laid-Open No. 2006-065399 is an example of a conventionally proposed image search technique based on the degree of similarity of feature points in images. According to the document, feature points similar to a pair of feature points randomly selected from a query image are obtained from a registered image in the image search based on the comparison of local features. A geometric transformation function between the query image and the registered image is then obtained based on the positional relationship between the pair of feature points. Corresponding feature points in the registered image existing at places after the transformation of feature points other than the pair based on the geometric transformation function are counted to determine the similarity.
In the process, if the feature points similar to the pair existing in the registered image do not correctly correspond to the pair of feature points randomly selected from the query, the process of applying the geometric transform to the feature points of the query image is significantly slow. Furthermore, if the feature points do not appear at the same locations in the registered image, the method of selecting the feature points is determined to be wrong, and a pair of feature points is randomly selected again from the query image.
More specifically, the image searching process of Japanese Patent Laid-Open No. 2006-065399 is a process in which the process from the selection of feature points to the geometric transformation is executed for a large number of times with an expectation that an appropriate pair of feature points is included in the processes. Therefore, a large amount of processing cost is required, and improvement is necessary. Furthermore, since the feature points are randomly selected, there is a problem that the reproducibility of a search result (including calculation with the same degree of similarity) is poor if the number of repetitions is small.
Furthermore, the process becomes slower when the processing result is processed to narrow down the search result based on the rotation angle or the enlargement/reduction ratio.